Under normal conditions, the rate of reactive oxygen species (ROS) production does not exceed the capacity of the tissue to catabolize them. Under certain conditions, however, ROS levels are raised beyond the capacity of these protective mechanisms (e.g., irradiation, environmental factors, iron loading, etc.) or when these mechanisms are faulty (e.g., genetic defects), the ROS can cause cellular and tissue damage leading to a variety of diseases and even death. Proteins, lipids, and DNA are all substrates for ROS attack. It has been calculated that for every 100 tons of oxygen consumed two tons form ROS. For every 1012 oxygen molecules entering a cell each day 1/100 damages protein and 1/200 damages DNA. It is this damage to DNA, proteins, and lipids that makes the ROS so dangerous, especially when the body's natural defenses are compromised.
It is suggested that oxidative stress plays an important role in aging. The levels of some antioxidant enzymes such as superoxide dismutase (SOD) and antioxidants such as uric acid, beta-carotene and vitamin E have a positive correlation with the life-span of biological species. Namely, the level of these antioxidant enzymes and antioxidants decreases from human to chimpanzee to mouse (Culter, Free Radicals in Biology, vol. 4: p. 371, 1984). One hypothesis is that cells are damaged by free radicals and the damaged cells cannot function properly, and that the accumulation of damages to cells leads to aging (Culter, Id.). Another hypothesis is that free radicals cause cells to dysdifferentiate from their proper state of differentiation, and that this dysdifferentiation of cells leads to aging and all kinds of age-related diseases (Culter, Id.). It is suggested that free radicals cause aging and age-related diseases. Free radicals have been implicated in stroke, ischemia-reperfusion, cardiovascular diseases, carcinogenesis and neurological diseases, including Alzheimer's disease, Parkinson's disease, dementia and Hodgkin's disease.
Complications of atherosclerosis, such as myocardial infarction, stroke and peripheral vascular disease account for half of the deaths in the United States. Arteriosclerosis begins with an injury to the endothelial cells and is associated with the proliferation of muscle cells inside the arteries. In the process of atherosclerosis, blood becomes thick and platelets, oxidized low density lipoprotein (LDL, the major lipid in LDL is cholesterol esters) and other substances begin to adhere to the walls of the arteries causing the formation of plaque. The oxidation of LDL is caused by free radicals.
It was first recognized in 1969 (McCully, Amer. J. Pathol. 56:111, 1969), and only recently rediscovered, that high level of plasma homocysteine is associated with an increased rate of death due to coronary artery disease (Nygard et al., N. Engl. J. Med. 24: 337, 1997; Graham et al., J. Am. Med. Assoc. 277:1775, 1997). Homocysteine injures endothelial cells, thereby causing atherosclerosis through a number of mechanisms, including the generation of hydrogen peroxide (H2O2). It has been reported that homocysteine decreased the bioavailability of NO (not its production) and impaired the intracellular antioxidant enzymes, especially the glutathione peroxidases (Upchurch et al., J. Biol. Chem. 272: 17012, 1997). The key event in the process is generation and presence of free radicals. The increase of hydrogen peroxide can be a cause or a result. Homocysteine causes the production of free radicals including superoxide (O2.−) which reacts with NO causing decreased bioavailability of NO and production of hydroxyl radical (.OH), or undergoes dismutation by SOD to produce hydrogen peroxide. Hydrogen peroxide is further converted to the reactive hydroxyl radical (.OH) through the Fenton reaction and the metal-catalyzed Haber-Weiss reaction. The free radicals produced as a result of these reactions will damage the antioxidant enzymes which prevents the detoxification of free radicals. It is clear that scavenging free radicals will prevent the toxic effects of LDL and homocysteine and results in the prevention of atherosclerosis.
Extensive research efforts, which include the use of antioxidant enzymes and antioxidants, have been made to counter the damaging effects caused by free radicals. Unfortunately, protein enzymes are too big to penetrate the cell wall and blood brain barrier. Antioxidants alone are not satisfactory for various reasons including the fact that they are consumed by free radicals and, thus, a large quantity is needed.
Several ROS exist. Diatomic molecular oxygen (O2) readily reacts to form partially reduced species, which are generally short-lived and highly reactive and include the superoxide anion (O2.−), hydrogen peroxide (H2O2), and hydroxyl radicals (.OH). The ROS are the byproducts of mitochondrial electron transport, various oxygen-utilizing enzyme systems, peroxisomes, and other processes associated with normal aerobic metabolism as well as lipid peroxidation. These damaging byproducts further react with each other or other chemicals to generate more toxic products. For example, hydrogen peroxide can be transformed to highly reactive hydroxyl radical (.OH) through the Fenton reaction and the metal catalyzed Haber-Weiss reaction:Fe2+H2O2→Fe3++.OH+OH−.OH+OH−→O2.−+H+ (Fenton reaction)O2.−+H2O2→.OH+OH−+O2 (Fe3+/Fe2+ catalyzed Haber-Weiss reaction)
Superoxide (O2.−) reacts with nitric oxide (NO) to form the toxic peroxynitrite (ONOO−) which further decomposes to release the hydroxyl radical (.OH).O2.−+NO→ONOO−→NO2.+.OH
Human beings have a defense system against toxic byproducts of metabolism including various enzymes such as superoxide dismutase (SOD), catalases and peroxidases, and various antioxidants such as vitamins (e.g., vitamin A, beta-carotene, vitamin C and vitamin E), glutathione, uric acid and other phenolic compounds. SOD catalyzes the conversion of superoxide into hydrogen peroxide and oxygen.2H++O2.−+O2.−→H2O2+O2 (catalyzed by SOD).
Hydrogen peroxide can be transformed by catalases and peroxidases to oxygen and water.2H2O2→O2+H2O (catalyzed by catalases and peroxidase)
Despite the high efficiency of the defense system, some of these damaging species escape. The escaped reactive oxygen species and their products react with cellular DNA, protein and lipid resulting in DNA damage and peroxidation of membrane lipids. The deleterious results caused by reactive oxygen species are termed oxidative stress which affects normal gene expression, cell differentiation (Culter, Free Radicals in Biology, vol. 4, p. 371, 1984; Culter, Ann. New York Acad. Sci. 621: 1, 1991) and leads to cell death. Oxidative stress is now considered to be responsible for many health problems like cardiovascular and neurological diseases, cancer and other aging-related diseases as well as the human aging process.
Cholesterol plaques cause hardening of the arterial walls and narrowing of the inner channel (lumen) of the artery. Arteries that are narrowed by atherosclerosis cannot deliver enough blood to maintain normal function of the parts of the body they supply. For example, atherosclerosis of the arteries in the legs causes reduced blood flow to the legs. Reduced blood flow to the legs can lead to pain in the legs while walking or exercising, leg ulcers, or a delay in the healing of wounds to the legs. Atherosclerosis of the arteries that furnish blood to the brain can lead to vascular dementia or stroke.
Cerebrovascular disease is the third leading cause of death after heart disease and cancer in developed countries. Five percent of the population over 65 is affected by a stroke. In the United States, stroke afflicts more than 500,000 people every year (Digravio, G., J. Am. Med. Assoc. 296:2923, 2006). Seventy to eighty-five percent of stroke injuries are due to ischemic stroke, which has major morbidity and a 15˜33% rate of mortality. Emerging treatments for acute cerebral ischemia include use of cytoprotective and thrombolytic agents (Phillips et al., Prog. Cardiovasc Dis., 50: 264, 2008). While cytoprotective treatments attempt to prevent cell death during ischemia and reperfusion, thrombolytic treatment depends on the early use of clot-lysing agents and the restoration of blood flow. Despite intensive research efforts, stroke remains one of the most devastating diseases in medicine. One reason for the ineffectiveness of the current stroke therapy is that no drug currently used functions effectively as a thrombolytic and cytoprotective agent at the same time.
Reduced blood supply to the heart muscle from coronary atherosclerosis leads to coronary heart diseases, which include heart attacks, sudden unexpected death, chest pain (angina), abnormal heart rhythms, and heart failure due to weakening of the heart muscle.
Most heart attacks occur as a result of coronary artery disease (CAD). CAD is the buildup over time of plaque on the inner walls of the coronary arteries. Occasionally, a section of plaque can break open, causing a blood clot to form at the site. A heart attack occurs if the clot becomes large enough to cut off most or all of the blood flow through the artery. If blood flow is not restored within 20 to 40 minutes, irreversible death of the heart muscle will begin to occur. Muscle continues to die for six to eight hours at which time the heart attack usually is “complete.” Treatment of heart attacks should include use of thrombolytic agents to dissolve the blood clot, following by the use of antioxidant to protect/salvage the damaged heart muscle cells.